Research has consistently shown diversity of vocabulary to be an important indicator of second language (L2) writing development as well as L2 writing performance. These studies underscore the importance of vocabulary to L2 writing. However, they provide little to indicate what kind of vocabulary learners of English may need to know in order to develop writing proficiency. This small-scale pilot study examined the relationships among vocabulary knowledge, vocabulary use, and L2 writing performance. The results suggest that accurate productive knowledge of high-frequency word families was associated with L2 writing performance. However, actual use of high-frequency word families was negatively associated with L2 writing performance. Based on the results, the authors present potential uses of lexical frequency information to help students develop (a) accurate productive knowledge of high-frequency word families and (b) a repertoire of low-frequency word families based on their communicative needs.